In recent years, since problems of global warming due to carbon dioxide emission by use of fossil fuel and radioactive contamination by nuclear power plant accidents and radioactive waste have become serious, interests in global environment and energy are growing. Under these circumstances, solar power generation that uses solar light as an inexhaustible and clean energy source, geothermal power generation using the geothermal energy, wind power generation using the wind power, and the like have been put into practice all over the world.
Solar power generation apparatuses using solar batteries employ various forms corresponding to output scales ranging from several W to several thousand kW. A representative system using a solar battery is a solar power generation system which converts (DC/AC-converts) a DC power generated by a solar battery into an AC power by an inverter or the like and supplies the AC power to a commercial power system.
Such a solar power generation system has a means for detecting a DC ground fault (resistance ground fault) of a solar battery to prevent any outflow of a current (to be referred to as a DC ground fault current hereinafter) due to the DC ground fault.
FIG. 9A is a view showing an example of DC ground fault detection in a solar power generation system using a transformer inverter as a system interconnection inverter. FIG. 9B shows an example of DC ground fault detection in a solar power generation system using a transformerless inverter as a system interconnection inverter.
In the example shown in FIG. 9A, a commercial power system 3 having one terminal grounded and the outputs of a system interconnection inverter (to be also simply referred to as an inverter hereinafter) 2′ are insulated from each other by a transformer in the inverter 2′. A voltage detector is connected between ground (earth) and a midpoint α between voltage-dividing resistors X1 and X2 connected in parallel to a solar battery 1. For example, when a DC ground fault occurs at a point β in the solar battery 1, the potential difference between ground and the voltage dividing point α is detected by the voltage detector, thereby detecting the DC ground fault.
As in the example shown in FIG. 9B, when the inverter 2 is a transformerless inverter, a DC ground fault current in a DC circuit causes unbalance in the DC circuit and AC circuit. With an emphasis on this fact, a DC ground fault current is detected on the basis of a zero-phase current in the DC circuit or AC circuit. This method is disclosed in, e.g., Japanese Patent Laid-Open No. Hei 11-136852.
As another method generally used, a DC ground fault is detected by measuring the insulation resistance of a solar battery using an insulation resistance tester.
However, the above conventional ground fault detection methods have the following problems.
A DC power supply in a power conversion system which converts a DC power from a DC power supply such as a solar battery into an AC power sometimes has a point where the DC voltage to ground (to be referred to as a DC voltage to ground hereinafter) becomes zero or almost zero. When a DC ground fault occurs near this point, the DC ground fault current is weak. Hence, the above method of detecting a DC ground fault current on the basis of a zero-phase current or the method of detecting a DC ground fault on the basis of a change in voltage to ground can hardly detect a DC ground fault at such a point.
Additionally, in the above method using an insulation resistance tester, since a high voltage must be applied across the DC circuit and ground, the operation of the inverter for converting the power must be stopped, resulting in low operation efficiency. In addition, since operation of measuring the insulation resistance is necessary, much labor and time are required.